Absolutely correct in WW1 & WW2 the soldier couldnt go home in the evening, and in the comfort of his own home, strip down and clean his rifle - we can.
Slide forward 50 - 100 years
We dont need oil in the bottle (and anyway, most of them leak and oil-soak your butt.
Its a great idea to have easily broken, small parts on hand when at the range or out hunting. a broken extractor can ruin your day, but if you've got one with you it can be fixed in minutes.
I have a 'repair kit' in an oil botle for each of my 303s - and yes - a broken case extractor easily fits in the bottle (but you do need to unscrew/remove the spoon) You can even fit in a small watchmakers screwdriver.
https://www.milsurps.com/images/impo...SCF00111-1.jpg